The goal of this research is to understand the developmental composition, function, and correlates of the vaginal microbiome in young Black women. This focus is justified because of the substantial sexual health and reproductive health disparities of Black women compared to women of other racial/ethnic groups. No research traces the evolution of the vaginal microbiome of Black women from adolescence into young adulthood. This is a developmental interval characterized by new vaginal hygiene practices (e.g., douching, tampon/pad use), changes in diet and physical activity, partnered sexual activity, hormonal and barrier contraceptive use, and sexually transmitted infections. The potential impact of the project would be to inform public health interventions intended to reduce sexual and reproductive health disparities among Black women. We propose to assemble a prospective cohort of 180 healthy Black, post-menarchal 14 year-old adolescent women. Vaginal swabs will be self-obtained monthly for up to 24 months. Vaginal microbiome composition will be assessed by phylogenetic analysis of 16S rRNA gene sequences. Vaginal microbiome function will be assessed by periodic assessment of vaginal pH and Nugent score, as well as quantification of salivary estrogen and vaginal glycogen. Nutritional correlates will be assessed with measurement of plasma 25- hydroxyvitamin D (25OHD) status. Sexual behavior and reproductive health correlates of the vaginal microbiome will be assessed using electronic daily diaries. STI correlates will be diagnosed prospectively with nucleic acid amplifications tests.